Erosion-inhibited functional fluids

ABSTRACT

THE ADDITION OF A SMALL AMOUNT OF PHOSPHONIUM OR AMMONIUM SALT OF THE FORMULA   (R)NNH+4NX- OR (R)NPH+4NX-   WHERE X- IS HALIDE OR ACETATE, R IS C1-C50 HYDROCARBYL WHICH MAY BE THE SAME OR DIFFERENT, AND N IS AN INTEGER FROM 0 TO 4, TO AN ENERGY-TRANSMITTING PHOSPHATE ESTER BASED FUNCTIONAL FLUID SEVERELY REDUCES THE STREAMING CURRENT OF THE FLUID. THE STREAMING CURRENT IS ASSOICATED WITH EROSION OF CONTROL VALVES IN THE FLUID-CARRYING SYSTEM.

3,790,487 EROSION-INHIBITED FUNCTIONAL FLUIDS Merwyn L. Burrous, El Cerrito, Calif., assignor to Chevron Research Company, San Francisco, Calif. No Drawing. Filed Nov. 5, 1971, Ser. No. 196,179 Int. Cl. C09k 3/02 US. Cl. 252-78 13 Claims ABSTRACT OF THE DISCLOSURE The addition of a small amount of phosphonium or ammonium salt of the formula where X- is halide or acetate, R is C -C hydrocarbyl which may be the same or different, and n is an integer from 0 to 4, to an energy-transmitting phosphate ester based functional fluid severely reduces the streaming current of the fluid. The streaming current is associated with erosion of control valves in the fluid-carrying system.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to fluid compositions which are useful for transmitting power in hydraulic systems. Specifically, it relates to power transmission fluids having a tendency to cause erosion of hydraulic systems and a newly discovered means of controlling such erosion.

Organic phosphate ester fluids have been recognized for some time as advantageous for use as the power transmission medium in hydraulic systems. Such systems include recoil mechanisms, fluid-drive power transmission, and aircraft hydraulic systems. In the latter, phosphate ester fluids find-particular utility because of their special properties which include high viscosity index, low pour point, high lubricity, low toxicity, low density and low flammability. Thus, for some years now, numerous types of aircraft, particularly commercial jet aircraft, have used phosphate ester fluids in their hydraulic systems. Other power transmission fluids which have been utilized include major or minor amounts of hydrocarbon oils, amides of phosphoric acid, silicate esters, silicones and polyphenyl ethers. Additives which perform special functions such as viscosity index improvement and foam inhibition are also present in these fluids.

The hydraulic systems of a typical modern aircraft contain a fluid reservoir, fluid lines and numerous hydraulic valves which actuate various moving parts of the aircraft such as the wing flaps, ailerons, rudder and landing gear. In order to function as precise control mechanisms, these valves often contain passages or orifices having clearances on the order of a few thousandths of an inch or less through which the hydraulic fluid must pass. In a number of instances, valve orifices have been found to be substantially eroded by the flow of hydraulic fluid. Erosion increases the size of the passage and reduces below tolerable limits the ability of the valve to serve as a precision control device. Early investigations indicated that the erosion was being caused by cavitation in the fluid as the fluid passed at high velocity from the highpressure to the low-pressure side of the valve. The incorporation of water into the hydraulic fluid was found to inhibit the erosion, but continuing experience shows that a significant erosion problem remains.

Recent studies indicate that certain valve erosions are associated with the electrokinetic streaming current induced by the high velocity fluid flow. Specifically, it has been found that if the alkali metal content of a phosphate ester fluid is reduced to 0.1-4.0 p.p.m., or preferably 0.2- 2.0 p.p.m., the streaming potential of the fluid can be reduced by a factor of as much as one-fifth, and that the 3,790,487 Patented Feb. 5, 1974 fluid then shows essentially no tendency to cause valve erosion.

Description of the prior art A study of the problem attributing valve erosion to the streaming current, induced by fluid flow is Beck et al., Corrosion of Servovalves by an Electrokinetic Streaming Current, Boeing Scientific Research Document D1-82- 0839 (September 1969). Efforts to control hydraulic valve erosion by treating the problem as one of cavitation in the fluid are described in Hampton, The Problem of Cavitation Erosion in Aircraft Hydraulic Systems, Aircraft Engineering, XXXVIII, no. 12 (December 1966). Netherlands, patent application 66/ 14,183 describes a typical phosphate ester hydraulic fluid to which water has been added in an attempt to suppress cavitation and thereby reduce valve erosion. US. Pat. 3,513,097 teaches the addition of water to hydraulic fluids to inhibit cavitation damage in the system. The text, Organophosphorus Compounds, by Kosolapotf (Wiley, New York, 1950), describes methods of preparing organophosphorous derivatives. Several patents describe phosphate ester hydraulic fluids, including US. Pats. 2,636,861, 2,636,862, 2,894,911, 2,903,428 and 3,036,012.

SUMMARY OF THE INVENTION It has now been discovered that phosphonium and ammonium salts when incorporated into power transmission fluids serve to reduce the electrical streaming current of the fluid which is believed to be associated with the erosion of control valves in the fluid transmission system. Specifically, in the art of hydraulic fluid formulation, the improvement comprising the inclusion of 0.01 to 5 weight percent of a phosphonium or ammonium salt in the hydraulic fluid. Preferably, the hydraulic fluid will contain 0.01 to 2 weight percent of an alkylphosphonium or alkylammonium salt and the fluid composition comprises organic phosphate esters.

DETAILED DESCRIPTION OF THE INVENTION The invention comprises the improvement in a power transmission fluid by the addition thereto of a small amount of phosphonium and/or ammonium salt.

Phosphonium salts The phosphonium salts are a class of true salts analogous to the quaternary ammonium salts. They are completely independent and distinct from the various organic esters of phosphorus-containing acids which are named as salts, such as the phosphates, phosphites, phosphonates and phosphinates. The phosphonium salts are also distinct from various other organophosphorous compounds such as the phosphines, biphosphines, phosphoranes and phosphine oxides.

The ammonium salts are of the general form (RMNHLX and the phosphonium salts are of the general form )n i-..X-

where X- is an organic or inorganic anion, R represents hydrocarbyl groups and n is 0-4. Not all the hydrocarbyl radicals need be the same in the phosphonium or ammonium salts, e.g., methyl diehylphosphonium chloride,

CH (C H PHCl.

X- may be a halide anion, a phosphate anion, an acetate anion, or some other inorganic or organic anion serving the purpose of providing a counter ion to the phosphonium ion. The phosphate counter ion may itself be an ester, for example, methyl trioctylphosphonium dimethylphosphate, propyl tributylphosphonium dioctylphosphate, etc.

The hydrocarbyl radicals, R, of the phosphonium or ammonium salt will, in general, contain from 1 to 50 carbon atoms and preferably from 1 to carbon atoms. Generic examples of these phosphonium or ammonium salts include alkylphosphonium phosphates, alkylphosphonium acetates, alkylphosphonium halides, alkyl-alkenylphosphonium phosphates, alkyl alkenylphosphonium halides, alkyl-alkenylphosphonium acetates, alkylphosphonium halides, alkyl-arylphosphonium phosphates, arylphosphonium acetates, alkenylphosphonium halides, etc. Specific examples of the phosphonium salts include trimethylphosphonium halide, tetrabutylphosphonium halide, methyl triethylphosphonium acetate, octyl trimethylphosphonium phosphate, methyl trioctylphosphonium phosphate, tetrabutylphosphonium acetate, ethyl tripropylphosphonium halide, phenyl tributylphosphonium acetate, etc., and the analogous ammonium salts.

Methods of preparation of the phosphonium salts are well known in the chemical literature. For example, the alkylphosphonium salts are prepared from a mixture of phosphonium iodide, alkyl halide and zinc oxide heated in a sealed container.

Fluid base The power transmission fluid of the present invention comprises a fluid base present in major proportion in which the phosphonium salts and other additives are contained. The fluid base in which the additives of this invention are employed include a wide variety of base materials, such as organic esters of phosphorus acids, mineral oils, synthetic hydrocarbon oils, silicate esters, silicones, carboxylic acid esters, aromatics and aromatic halides, esters of polyhydric material, aromatic ethers, thioethers, etc.

The phosphate esters which are the preferred base fluid of the present invention have the formula wherein R R and R each represent an alkyl or aryl hydrocarbon group. (As used herein, aryl includes aryl, alkaryl, and aralkyl structures and alkyl includes aliphatic and alicyclic structures.) All three groups may be the same, or all three different, or two groups may be alike and the third different. A typical fluid will contain at least one species of phosphate ester and usually will be a mixture of two or more species of phosphate esters.

The phosphate esters will each have a total carbon content of 3-36 carbon atoms. Individually alkyl groups will usually have 1-12 carbon atoms, while individual aryl groups will usually have 6-12 carbon atoms. Preferred esters contain 12-24 total carbon atoms, preferably alkyl groups, 4-6 carbon atoms, and preferred aryl groups, 6- 9 carbon atoms. The alkyl groups may be straightor branched-chain, with straight-chain, such as n-butyl, preferred. Similarly, the alkyl substituents in alkylaryl structures may also be straightor branched-chain. Generic examples of the phosphate esters include trialkyl phosphates, triaryl phosphates and mixed alkylaryl phosphates. Specific examples include trimethyl phosphate, tributyl phosphate, dibutyloctyl phosphate, triphenyl phosphate, phenyl dicresyl phosphate, methyl diphenyl phosphate, dibutylphenyl phosphate, tricresyl phosphate, etc.

In practice, phosphate ester fluid base generally contains several phosphate esters mixed together. Usually, one particular ester or several closely related esters will predominate. In a preferred type of fluid, the phosphate ester portion contains only trialkyl and triaryl phosphate esters, with the trialkyl phosphate esters predominating. Typically, the phosphate ester portion of this fluid will consist of 70-99 weight percent, preferably 80-92 weight percent trialkyl phosphate esters, with the remainder triaryl phosphate esters. The phosphate ester portion is normally 75-95 weight percent of the total fluid and preferably -95 weight percent.

Additives The power transmission fluids of the present invention generally contain a number of additives which in total comprise 5-25 weight percent of the finished fluid. Among these is water, which may be added (as is described, for instance, in the aforementioned Netherlands patent application), or often becomes incorporated into the fluid unintentionally. Such incorporation can occur when a hydraulic system is being refilled and is open to the atmosphere, particularly in humid environments. Unintentional incorporation of water may also occur during the manufacturing process of a phosphate fluid. In practice, it is recognized that water will be incorporated into the fluid and steps are taken to control the Water content at a level in the range of 0.1-1 weight percent of the whole fluid. It is preferred that the water content be in the range of 0.1-0.8 weight percent and more preferably 0.2-0.6 weight percent.

Hydrolysis inhibitors to retard corrosion are often added to hydraulic fluids. They include various epoxides such as the glycidyl ethers described in US. Pat. 2,636,- 861. Typical epoxide compounds which may be used include glycidyl methyl ether, glycidyl isopropyl ether, styrene oxide, ethylene oxide, and epichlorohydrin. Hydrocarbon sulfides, especially hydrocarbon disulfides, such as dialkyl disulfide, are often used in combination with the epoxide compounds for additional corrosion suppression. Typical hydrocarbon disulfides include benzyl disulfide, butyl disulfide and diisoamyl disulfide. A particularly preferred class of epoxide hydrolysis inhibitors are those containing two linked cyclohexane groups to each of which is fused an epoxide (oxirane) group. Illustrative are those in which the linking structure contains a carboxylic acid ester group.

The hydraulic fluid normally contains 2-10 weight percent, preferably 5-10 weight percent, of one or more viscosity index improving agents such as alkyl styrene polymers, polymerized organic silicones, or preferably, polyisobutylene, or the polymerized alkyl esters of the acrylic acid series, particularly acrylic and methacrylic acid esters. These polymeric materials generally have a number average molecular weight of from about 5,000 to 300,000.

Measurements It has recently been found that the rate of valve erosion in aircraft hydraulic system valves varies with the electrical streaming potential of the hydraulic fluid passing through the valve. Streaming potential is defined on pages 4-30 of the Electrical Engineers Handbook, by Fender and Del Mar (New York, Wiley, 1949). It is the EMF created when a liquid is forced by pressure through an orifice and is a function of factors such as the electrical properties and viscosity of the liquid, the applied pressure, and the physical characteristics of the orifice. Since the streaming potential is dependent on several factors, it is found that the streaming potential measurement of a given fluid on a given apparatus at a given time will vary over a small range. For this reason, the ordinary practice is to select as a standard a fluid which is considered to have acceptable erosive characteristics. Each day the apparatus is calibrated by measuring the streaming potential of the standard fluid and then comparing the streaming potential of the test fluids against this standard. The apparatus used to measure streaming potential is described in detail in the Beck et al. report referred to above. Measurements are taken at room temperature with the fluid pressure adjusted to 1,500 p.s.i. For convenience, the streaming potential detected by the apparatus is impressed across a standard 100,000-ohm resistor to obtain a resultant current, which is reported as the streaming current. Table I illustrates the reduction in streaming current obtainable from the inclusion of small amounts of phosphonium salts in a hydraulic fluid.

TABLE I Concen- Streaming Phosphonium Salt tration Fluid current 3 None A 3 0. 7 Tetrabutylphosphonium chloride.. 0.1 A 5 0. 2 D 0. 25 A 0.08 Do 1. 0 A 8 0 Methyltrioctylphosphonimn dimethyl phosphate 0. 1 A B 0. 8 D0 0. 25 A a 0.6 1.0 A 3 0.02 B 4 1.0 Tetrabutylphosphoninm chloride. 0. 25 B 4 0. 04 'letrabutylphosphonium acetate 0. 1 A a 0. 25 Do 0.25 A a 0.02 None C 5 1. 9 Tetrabutylphosphonium acetate 0. 5 C 5 0.02

1 Concentration as weight percent of phosphonium salt of total fluid.

In microamperes.

Fluid A is composed of 90.7 weight percent of mixed alloyl and aryl phosphate esters, 6.8 percent polyalkyl acrylate, 2.5 percent cycloaliphatic epoxi de, 0.001 percent antiioemant and 0.4-0.6 percent water.

4 Fluid B is composed of 93.3 weight percent of alkyl-aryl phosphate ester, 5.6 percent polyalkyl acrylate, 1.1 percent epoxidized soybean oil, 0.05 percent C -polyol, 0.001 percent antiioamant and 0.45 percent water.

5 Fluid 0 has the same composition as Fluid A with the addition of 25 p.p.m. sodium.

Reductions in the streaming current of up to 100 percent have been obtained with as little as 1 percent of phosphonium salts in a phosphate ester fluid base. A variety of phosphonium salts have been shown to be effective in reducing the streaming current and there is no reason to believe that this eflfect is limited to the illustrated salts. Thus, the phosphonium salt-containing fluid is a markedly superior hydraulic fluid, particularly for aircraft use as proven by the following data.

TABLE II Concen- Streamin Salt tration 1 Fluid current None A 0. Calcium phenate 0. 2 A 0.9 Sodium sulfonate 0. 5 A 2. 2 Sodium suliosuccinate 0. 5 A 1. 4 t-Octyl amine salt of butyl acid 0.1 1. 0 A 2. 6, 1. 5

phosphate. Ethoxylated quaternary am- 0. 5, 1. 0 A 0. 7, 0.6

monium chloride. Ethoxylated aliphatic diamine..- 0. 5 A 1. 35 LiCl 21 p.p.m. Li A 3.3 LiOl- 7 p.p.m. Li A 2.1 LiButyl phosphoric acid. 7 p.p.m. Li A 1.3 Butyl phosphoric acid 0. 02,2. 0 A 0. 95, 0.2 Tetra butyl ammonium chloride 0.05, 0.25 A 0. 4, .19 DL leucine 0. 05 A 1. 2

1 Cdoncentration as weight percent of total fluid, unless otherwise note 2 In microamperes.

5 Fluid A is composed of 90.7 weight percent of mixed alkyl and aryl phosphate esters, 6.8 percent polyalkyl acrylate, 2.5 percent cycloaliphatic epoxide, 0.001 percent antiioamant and 0.4-0.6 percent water.

9 In microamperes.

I In units of milliliters per minute per hour.

4 A commercial aralkyl phosphate ester hydrauhc flu d.

5 An experimental hydraulic fluid containing a 9/1 mixture of trialkyl phosphate ester/trianyl phosphate ester. l

6 A commercial phosphate ester hydrauhc fluid taken from an airline fleet with known valve erosion problem.

Table II illustrates the reduction or increase in streaming currents obtained when small amounts of a variety of nonphosphonium salts are added to a commercial hydraulic fluid. This data shows streaming currents which vary from good (0.1 microampere) for ammonium salts to very poor (3 microamperes).

Table III illustrates the remarkable reduction in erosion of control values which results from the addition of a small amount of a phosphonium salt to several commercial hydraulic fluids. The correlation between erosion and streaming current is also illustrated by Table III. The valve used in these tests was a nitrided nitralloy steel hydraulic control valve similar to those used in aircraft and of hardness Rockwell C-60. The valve gap was set at 75 microinches. The fluid pressure was 3,000 p.s.i. and the fluid sump temperature was 8=0100 F. The leak rate was measured periodically in tests running from 24 to or more hours. The increase in the leak rate (which is related to the rate of erosion) is given as the average increase in leak rate per hour over the course of the test. Since the leak rate is in units of milliliters per minute, the average increase in leak rate is in units of milliliters per minute per hour. Table III shows better than a twenty-fold reduction in the average leak rate per hour upon the introduction of 025-05 weight percent of a phosphonium salt. This is a result with important implications for the functional fluid art.

The above data and examples are intended to be illustrative. The scope of the invention is to be considered limited only by the following claims.

I claim:

1. An erosion inhibited phosphate ester based power transmission fluid comprising a major amount of a phosphate ester and from 0.01 to about 5 percent by weight of a phosphonium or ammonium salt of the formula where X- is selected from the group consisting of halide and acetate, R represents a (B -C hydrocarbyl group which may be the same or different, and n is an integer from 0 to 4.

2. The power transmission fluid of claim 1, in which the phosphate ester is a mixed alkylaryl phosphate.

3. The power transmission fluid of claim 1, in which the phosphate ester is a mixture of trialkyl phosphate and triaryl phosphate.

4. The power transmission fluid of claim 3, in which the trialkyl phosphate is tributyl phosphate and the triaryl phosphate is tricresyl phosphate.

5. The power transmission fluid of claim 1, in which the phosphonium or ammonium salt is an alkyl phosphonium or alkyl ammonium salt.

6. The power transmission fluid of claim 5, in which the alkyl group(s) of the alkylphosphonium or alkylammonium salt contain(s) from 1 to 10 carbon atoms.

7. A power transmission fluid according to claim 6, in which the alkylphosphonium or alkylammonium salt is selected from the group consisting of alkylphosphonium or alkylammonium halides and alkylphosphonium or alkylammonium acetate.

8. The power transmission fluid of claim 7, in which the alkylammonium salt is tetrabutylammonium chloride and the alkylphosphonium salt is chosen from the group consisting of tetrabutylphosphoniurn chloride and tetrabutylphosphonium acetate.

9. The method of inhibiting the erosion damage caused by a phosphate ester based power transmission fluid containing a major amount of a phosphate ester, which comprises maintaining in said fluid, by addition, a concentration of 0.01 to 5 percent by weight of a phosphonium or ammonium salt of the formula where X- is selected from the group consisting of halide and acetate, R represents a C C hydrooarbyl group which may be the same or different, and n is an integer from 0 to 4.

10. The method of claim 9, in which the phosphate ester is a mixed alkylaryl phosphate.

11. The method of claim 10, in which the mixed alkylaryl phosphate is dibutylphenyl phosphate.

12. The method of claim 9, in which the phosphate ester is a mixture of trialkyl phosphate and a triaryl phosphate.

13. The method of claim 12, in which the trialkylphosphate is tributyl phosphate and the triaryl phosphate is tricresyl phosphate.

15 RICHARD References Cited UNITED STATES PATENTS 6/1943 Rosen 25249.9 X 10/1944 Rosen 25249.9 X 3/1959 Semmens et a1. 25249.9 X 7/1960 Semmens et al. 252-49.9 3/ 1967 Hotten 25278 X 6/1968 Capowski et al. 252389 X 7/1971 Godfrey et al. 25278 12/1972 Gentit et al. 25278 FOREIGN PATENTS ll/ 1957 Great Britain.

D. LOVERING, Primary Examiner U.S. Cl. X.R. 

